PM2 Log Watcher -- Telegram Error Notifier

A lightweight Node.js utility that monitors PM2 error logs and sends real-time notifications to Telegram whenever new errors appear.
Built for servers running many PM2 apps, making it easy to catch issues instantly without manually checking logs.

🔍 What This Tool Does

• Monitors the newest PM2 *-error.log file that was modified today
• Detects only newly added error lines (not the whole file)
• Remembers the last read position using an offset file
• Sends formatted alerts to Telegram, including:
  • App name\
  • Hostname\
  • Timestamp\
  • Error lines (auto-truncated if too long)
• Auto-creates cache directory for offsets
• Uses .env for configuration
• Safe for long logs (cuts off at ~4000 characters to avoid Telegram limit)

🧩 How It Works

1. Every interval (default 5 seconds), the watcher:

   • Looks at PM2 error logs (*-error.log)
   • Picks the newest log modified today
   • Reads only the section added since the last scan
   • Filters lines containing "error"
   • Sends the first ~4000 characters to Telegram

2. Offset is stored in:

   /opt/pm2-watcher/cache/{app}-{YYYY-MM-DD}.offset
   

3. If log rotates or shrinks, offset resets automatically.

⚙️ Environment Variables

Configure via .env:

LOG_DIR=/root/.pm2/logs/
CACHE_DIR=/opt/pm2-watcher/cache/
BOT_TOKEN=YOUR_TELEGRAM_BOT_TOKEN
CHAT_ID=YOUR_CHAT_ID
INTERVAL_MS=5000

🚀 Run

node watcher.js

Or with PM2:

pm2 start watcher.js --name pm2-watcher

📦 Dependencies

• Node.js (ESM)
• node-telegram-bot-api\
• dotenv

🛡️ Why Use This?

• No need to SSH into the server to check PM2 logs\
• Instant Telegram alerts for new errors\
• Zero-downtime monitoring\
• Simple, fast, and minimal resource usage\
• Maintains daily offsets automatically